For the purpose of cracking hetero-atom compounds, asphaltenes and residual carbon precursors which are contained in hydrocarbons or for changing their properties or compositions, the hydrotreatment of hydrocarbons in the presence of a catalyst has been in wide use in the art. To this end, catalysts of diversified properties and compositions are used, depending upon the nature of the feed oil, the purpose of the treatment or the reaction conditions. As a matter of course, the catalyst which is used for the hydrocracking treatment deteriorates as the reaction proceeds. The causes of deterioration which are common to almost all of the hydrotreatment catalysts include the deposition of carbonaceous materials on the catalysts in addition to the influences of the contaminants such as metals and sulfur components which are contained in the feed oil. The mechanism of the deposition of the carbonaceous materials on the catalyst or the structure and composition of the deposited carbonaceous materials are hardly known except the fact that the deposition occurs in a relatively large amount when a catalyst with a higher solid acid content, that is to say, with a low hydrogenation power, is used under high temperature and low hydrogen pressure conditions. For instance, the deposition of carbonaceous materials is as low as 10% by weight of the catalyst in the treatment of a feed oil free of a residual oil but it exceeds 40-50 wt % and in some cases deposition in excess of 200 wt % is experienced in the treatment of residual oils. The catalyst which has been deteriorated by the deposition of carbonaceous materials is usually used again after regeneration of the catalyst by the removal of the carbonaceous materials, in some cases, recovering the deposited metals after removal of the carbonaceous materials. However, these methods are not advantageous technically.
The hydrocracking of hydrocarbons, especially, of heavy hydrocarbons, requiring a high reaction temperature, inherently involves a problem that the catalyst is deteriorated within a short time period due to the increased deposition of carbonaceous materials on the catalyst. Therefore, it becomes necessary to replace or regenerate the catalyst frequently. With regard to the replacement or regeneration of the catalyst in the reaction system, there have thus far been proposed various methods. The representatives of them are the methods using fluidized catalyst such as Varga process, slurry-catalyst process and suspension-catalyst process which are widely practiced by virtue of many advantages resulting from the use of a fluidized catalyst of finely divided solid particles. The advantages which accrue from the use of a catalyst in the form of fluidized finely particulate solid particles include:
(1) The catalyst is freely movable within the reactor and the catalyst which has been deteriorated due to coking or metal deposition can be withdrawn along with the product oil, so that the reactor is less susceptible to blocking and it is possible to carry out the treatment under a lower hydrogen pressure and at a higher temperature as compared with other methods;
(2) Consequently, the chemical hydrogen consumption as well as the equipment cost is reduced;
(3) The catalyst which is in the form of fine particles is free of the influences on its activity of the average pore diameter or other factors of its physical structure, and almost immune from the catalytic deterioration owing to metal deposition; and
(4) An extremely broad range of catalysts are applicable, including such a cheap catalyst as a ground, degenerate catalyst after use in the conventional fixed bed.
However, notwithstanding the foregoing advantages, the use of finely particulate catalyst still has problems to be solved, as follows.
(1) An extremely large amount of catalyst is consumed since it is not regenerated partly because of its cheapness and the difficulties of its regeneration. Therefore, in spite of the cheapness, there arises a problem of a large expense required for the catalyst even in the treatment of feedstock oils of lower grade;
(2) The discard of the spent catalyst brings about a new problem of environmental pollution due to the heavy metals deposited thereon;
(3) Even if, in order to solve the foregoing problems, the spent catalyst is separated from the product oil for reuse, the activity of the catalyst is considerably deteriorated due to a large quantity of coke deposition on the catalyst. Further, the catalyst is apt to cohere, resulting in the precipitation within the reactor or clogging of the reactor;
(4) Considerable difficulties are practically involved in the separation of the finely particulate catalyst from the product oil;
(5) When the catalyst separated from the product oil is subjected to oxidative roasting for reuse, the recovery of the catalyst is difficult due to cohesion or fusion of the individual particles during the roasting treatment, and the workability is extremely poor.
In an attempt to solve the above-mentioned problems which are encountered in the hydrocracking of hydrocarbons with the use of a dispersed or suspended micro-particulate solid catalyst (hereinafter referred to as "slurry catalyst" for brevity), there have thus far been made various proposals in the art. U.S. Pat. Nos. 3,622,495 and 3,622,498 disclose a process for converting a heavy oil with high asphaltene and vanadium contents into a light oil with reduced asphaltene and vanadium contents through a hydrotreatment using a micro-particulate vanadium sulfide catalyst, in which the product oil is separated into light and heavy fractions, and the catalyst-containing heavy fraction is recycled to the step of the hydrotreatment. According to this process, it is possible to obtain a light oil substantially free of asphaltenes by the use of the highly active catalyst. However, it necessitates to extract a large quantity of heavy oil which contains the spent catalyst and to add fresh catalyst in supplement therefor since the catalyst is deteriorated by the cyclic use. With regard to the cyclic use of such vanadium sulfide catalyst, U.S. Pat. Nos. 3,645,912 and 3,635,838 describe a process for removing carbonaceous materials and contaminant metals from the catalyst to be recycled, in which the spent catalyst is subjected to reaction with elementary sulfur at 500.degree.-1000.degree. C. thereby removing the carbonaceous materials as carbon bisulfide before eliminating the metals with use of a mineral acid. This process has an advantage in that the vanadium sulfide can be regenerated into vanadium tetrasulfide in the regenerative step to serve as a catalyst with high activity, but involves the problems as encountered in the oxidative treatment of a spent catalyst to be recycled.
Japanese Laid-Open Patent Specification No. 53-78203 discloses a hydrotreatment process using as a catalyst a pulverized, spent catalyst from a fixed bed type hydrotreatment system, in which the catalyst is separated from the product oil for recycling after regeneration by roasting. Because of the inclusion of the step of oxidative roasting, this process also fails to solve the problems in the slurry-catalyst process as mentioned hereinbefore. In this connection, Japanese Patent Publication No. 49-16522 and Japanese Laid-Open Patent Specification Nos. 55-16188, 55-131094 and 55-161885 disclose a process employing a combination of a primary hydrotreatment step using a slurry catalyst and a secondary hydrotreatment step using an ebullated catalyst bed or a fixed catalyst bed. Such a process can eliminate at least part of the drawbacks of the hydrotreatment using an ebullated or fixed catalyst bed but gives no consideration to the solution of the problems connected with the cyclic use of the slurry catalyst.
As another hydrotreatment method using the fluidized catalyst, there is employed an ebullated bed of a particulate catalyst. This method uses a catalyst of a larger particle size as compared with the above-mentioned slurry-catalyst process so that it is distinguished from the latter process in that it permits easier separation of the catalyst from the product oil. However, it resembles the slurry-catalyst process in that the catalyst which is dispersed in fluidized state in the hydrocarbon oil can be treated under high temperature and low hydrogen pressure conditions and that continuous extraction or replacement of the catalyst is possible. Nevertheless, the ebullated bed process also involves a number of difficult problems for the regeneration of the spent catalyst. More specifically, neither the solvent-washing method nor the oxidative roasting method which is usually resorted to for the regeneration of the ebullated bed catalyst is satisfactory. For example, the solvent-washing method has the following drawbacks.
(1) Only part (oil-soluble components) of the carbonaceous materials deposited on the catalyst is removed in the washing step;
(2) It is very difficult to remove by a normal washing operation the carbonaceous materials which have accumulated in the catalyst in a large amount; and
(3) Complete recovery of the solvent used in the washing operation is difficult.
On the other hand, the oxidative roasting which permits efficient removal of the carbonaceous material also has a number of problems, as follows.
(1) The oxidative roasting cannot be carried out in the reactor and requires the transfer of the spent catalyst. However, such transfer involves a considerable technical difficulty in handling the spent catalyst which is in reduced state and thus very susceptible to oxidation;
(2) The deposited metals on the catalyst which have been converted from sulfides to oxides upon oxidative roasting of the catalyst require a reducing and/or sulfurizing treatment for reuse. It is difficult to recover the deposited metals on the catalyst as elemental metals because they are present in the form of stable oxides;
(3) The oxidation reaction proceeds at an extremely high velocity even at a low temperature and generates considerable heat to make the control of the reaction difficult.
As explained hereinbefore, the conventional fluidized-catalyst hydrotreatment processes, including the slurry-catalyst and ebullated-bed-catalyst processes, give no solution to the fundamental problems connected with the regeneration of the spent catalyst.
Therefore, it is a primary object of the present invention to provide an improved process for hydrocracking hydrocarbons in the presence of a fluidized catalyst under hydrogenation reaction conditions, in which the coke level of the catalyst is maintained below a predetermined value.
It is another object of the present invention to provide a process in which the spent catalyst extracted from the reaction system is recycled to the hydrocracking step after a novel regenerative treatment.
It is still another object of the present invention to provide a novel process including a regenerative treatment for removing toluene-insoluble carbonaceous materials which are deposited on the spent catalyst extracted from the reaction system.
It is a further object of the present invention to provide a novel two-stage hydrotreatment process for hydrocracking hydrocarbons at a coke level below a predetermined value.
In order to achieve the above-mentioned objects, the present inventor has conducted an extensive research and as a result found that the toluene-insoluble carbonaceous materials which are deposited on the catalyst may be solubilized and removed by hydrotreatment of the spent catalyst extracted from the reaction system so that the hydrotreated spent catalyst may again exhibit a high hydrocracking activity.